Alternating current control systems ordinarily use solid state control logic to control conventional relays that switch power to prime movers to control physical variables. Unfortunately, conventional relays generate radio frequency interference (RFI) that causes logic errors in digital control circuitry. See D. Shoup, Radio Frequency Interference, Instruments & Control Systems, July 1974, p-63-66. Voltage output of a sensor in a conventional control circuit is fed to a difference amplifier. Output from this amplifier is fed to a separate comparator and combined with an electrically generated signal from an associated generator to yield time proportionate output pulses. Time proportionate, as used in this context, means duration of output pulses from the comparator is a function of the magnitude of the error signal introduced to the comparator. See W. Sahm, Solid State Relays aren't all Alike, Electronic Products Magazine, July 15, 1974, p-50-58. Output of each of these comparators is then used to switch a relay, or other mechanical switching means. It is well-known in the art to use these separate comparators to control triacs, or other solid state switching units such as solid state relays. See D. R. Grafhom, et al. SCR Manual, 5th Edition 1972 (General Electric) p-327-332. Conventional control circuits generally compare a sensed voltage to a reference voltage and outputs the difference between these voltages as an error signal. The error signal controls a solid state switch that in turn controls a prime mover. Normally two difference amplifiers are required. Signals generated as outputs by the difference amplifiers act to control two solid state or mechanical switches that, in turn, drive the prime mover between its extreme operational limits. Conventional time proportionation is accomplished by inserting a comparator in the circuit between the difference amplifier and the switch. The comparator has another input from an electrical signal generator. The comparator produces an output pulse only if the difference between the amplitude of the electrical signal and the amplitude of the output of the difference amplifier, the error signal, is either positive or negative, as may be determined by the designer of the system. Because the signal generator can produce any desired amplitude and waveform, the amplitude of the output of the generator is a known function of time. The magnitude of the error signal outputed by the difference amplifier will be a function of the difference between the amplitude of the reference signal and the sensor signal. Thus, the error signals magnitude is a function of the amount of correction needed to conform the physical variable to the desired state. The comparator, having these two signals as input, produces output pulses of duration dependent upon the output of the generator, and the amplitude of the output error signal of the difference amplifier. In a conventional systems, the output of these two comparators will each be fed to a solid state switch, which will control a prime mover and the prime mover will alter the controlled physical variable.
Such a conventional system only increases or decreases the physical variable's state and performs that function at a fixed rate. See O. P. Knudsen, Solid-state approaches to cooking-range control, The RCA Solid State 1974 Data Book, RCA, p-463-494. Generally, such a system uses a triac controller and a triac that in turn controls the prime mover. Such triac controllers and triacs are well-known. See O. Hitchkiss, et al., Relays don't have to Click or Chatter, Instruments and Control Systems, July 1974, p-55-58 and E. Dowdell, The Solid State Relay, Electronic Products Magazine, July 15, 1974, p-47-50. Generally it is desirable to switch triac on and off very close to zero volts and then increase the voltage through the triac. This procedure avoids problems associated with sudden switching of high voltage, high current loads. For a discussion of the state of the art see, R. Hook, Application of the M A742 Triagac: A zero crossing AC trigger, Fairchild (1970), p-9-14.
The cited art well describes the state of the art of solid state electronic switching. Most conventional systems use a hybrid of solid state logic control circuits and electromechanical relays for power control. These hybrids are subject to RFI problems as well as to maintenance problems associated with all electromechanical devices having moving parts. The most sophisticated solid state systems known to the inventor, other than the present invention, utilize zero voltage triac switching and time proportionating. These systems operate to vary the AC prime mover of the controlled physical variable only in one direction. Further, this variance is accomplished at a fixed rate. Finally, all conventional systems that use difference amplifiers to generate error signals are dependent on input of both a sensor voltage and a reference voltage. Failure or interruption of either of these voltages causes the difference amplifier to generate an extremely large error signal and the control system to actuate the prime mover toward an extreme state of the variable under control.